Originally Posted by Brother Oni
A musician pointed out to me that the half of all life disappearing went full meta, with the Avengers theme playing during post-Snap in the second movie sounding extremely somber and subdued, because half the instruments in the orchestra were missing.
I never picked up on that, but very cool. It does seem a certain amount of thought was put into it.

Originally Posted by Mechalich
The Snap is stupid…. It will never be possible to integrate it forever and it's just going to be an albatross around MCU worldbuilding until the multiverse renders it irrelevant.
At this point the Snap is pretty much baked into the heart of the MCU. The first ten years were leading up to it, and whether you consider that a bug or a feature is a matter of personal taste. I happen to think the Infinity Stones are completely ridiculous, and I wish the MCU had taken a different route, but here we are.

And speaking of bugs….

Originally Posted by Mechalich
Many insect groups reproduce their entire population every year, while long lived trees or whales might take centuries to recover….
Ecologists call this variation the difference between r- and K-selection, with larger and usually longer-lived organisms considered K-selected, and the small-bodied, fast-reproducing species considered r-selected.

And in fact, humans aren’t in the middle; we’re relatively large for mammals, quite long-lived, and most importantly we typically only have one offspring per birth, so we are squarely K-selected.

Originally Posted by Mechalich
If that includes half of all plant life, guess what happens to the Earth: everyone dies.
Not necessarily. As usual, ecological effects are far more complex than people (or demented alien warlords) often realize.

In the vertebrate world, predator populations typically take longer to recover from catastrophes than prey, so most predator species would probably crash early on. This would lead to a quick bounceback from prey species, so five years later there may actually be a larger population of deer and elk, even allowing for hunting pressure from remaining humans.

This would have major impacts on species composition of forests and fields. As just one example, in the US Mid-Atlantic I could see this driving understory species composition towards dominance by pawpaw, barberry and spicebush, since those are common species which deer won’t eat. This might actually make some local forests more productive for humans, since pawpaws are an excellent food resource.

It should be obvious that I’m not saying the post-Snap human population will be living high on venison and pawpaw bread, but there will be countless unexpected local variations which might provide better-than-starvation options for some areas.

But there would also be some massive unintended consequences. On a broader scale, the disappearance of half the trees from the Amazon basin would have two major effects. First, much more sunlight would be reaching the forest floor. Under ordinary circumstances, when a large tree falls, it creates an empty space in the canopy, called a light gap, which allows sunlight to stimulate the germination of seedlings from the seed bank under the light gap. This sets off a race for the canopy among fast-growing seedlings. In typical rainforest, light gaps account for maybe 2-3% of total surface area; but if that’s suddenly expanded to 50%, then there will be a tremendous pulse of new growth throughout the Amazon, with the surviving trees rising above a dense shaggy carpet of seedling thickets.

Unfortunately, without the canopy to intercept tropical rainfall, and with half as many full-sized trees to absorb the water through their roots, there will be far greater rates of erosion and soil runoff, from already thin and nutrient-depleted soils. Rivers throughout the Amazon will run thick red from soil erosion, and whether or not the masses of fast-growing seedlings can blunt that erosion is a real question. Whatever soil is lost to the rivers will eventually roll downstream to the mouths of the Amazon, past Marajó and into the Atlantic, where it will settle on top of the existing sediment fan, and will be lost to the Amazon ecosystem forever. This may cause permanent soil depletion and, at the least, result in a severely species-poor Amazon biome.

The only silver lining here is that the sudden intense pulse of growth from seedlings—assuming they can derive enough nutrients from the washed-out soil—will pull gigatons of CO2 from the atmosphere, not just in the Amazon but all around the world. Between this and the probable large-scale collapse of industrial emissions, global climate change may tip from warming to cooling, which will have its own raft of unintended consequences—although the sudden return of 3.5 billion humans may lead to a resumption of emissions.

So it’s an immensely complex situation, and the post-Snap world makes a fascinating setting for just that reason.